President Obama, Secretary of Defense Hagel, and Chairman of the Joint Chiefs of Staff U.S. Army General Dempsey attend remembrance ceremonies for 9/11 at the Pentagon 9/11 Memorial in Washington

GARY CAMERON / Reuters

(L-R) U.S. President Barack Obama, Secretary of Defense Chuck Hagel, and Chairman of the Joint Chiefs of Staff U.S. Army General Martin Dempsey attend remembrance ceremonies for 9/11 at the Pentagon 9/11 Memorial in Washington September 11, 2013. The large flag in background marks the point of impact where a jetliner crashed into the Pentagon 12 years ago today.

(L-R) U.S. President Barack Obama, Secretary of Defense Chuck Hagel, and Chairman of the Joint Chiefs of Staff U.S. Army General Martin Dempsey attend remembrance ceremonies for 9/11 at the Pentagon 9/11 Memorial in Washington September 11, 2013. The large flag in background marks the point of impact where a jetliner crashed into the Pentagon 12 years ago today. (GARY CAMERON / Reuters)

(L-R) U.S. President Barack Obama, Secretary of Defense Chuck Hagel, and Chairman of the Joint Chiefs of Staff U.S. Army General Martin Dempsey attend remembrance ceremonies for 9/11 at the Pentagon 9/11 Memorial in Washington September 11, 2013. The large flag in background marks the point of impact where a jetliner crashed into the Pentagon 12 years ago today.